The menstrual cycle occurs monthly from puberty until menopause. Once a month, a follicle in the ovary matures and ovulation occurs. An egg is released into the fallopian tube. If sperm are present, the egg may become fertilized. The fertilized egg passes to the uterus, implants and an embryo develops. If the egg is not fertilized, it will degenerate and the uterine lining, which is not needed for an embryo to implant, breaks down and is shed during menstruation. Hormones play a big role in this process. The ovaries produce estrogen and progesterone. During the follicle stage, an egg matures and the follicle releases the hormone estrogen, which stimulates the uterine lining to thicken. This stage takes 14 days. Ovulation occurs approximately fourteen days after the last menstrual period. Next, the egg enters the oviduct. Once ovulation occurs, corpus luteum forms over the ruptured follicle in the ovary. It secretes the hormone progesterone. It helps the uterine lining thicken more in anticipation of a fertilized egg implanting in the uterus. This stage takes about 12 more days. Next, if the egg is not fertilized, the uterine lining breaks down and menstruation occurs. If the egg is fertilized, it will implant in the lining and a pregnancy will occur.